The olefinic thermoplastic elastomer is now widely used in, for example, automobile parts, industrial machine parts, electronic and electrical equipment parts and building materials as an elastomer capable of energy saving and resource saving, especially, as a substitute for vulcanized rubber.
The olefinic thermoplastic elastomer can be divided into the crosslinked type and the noncrosslinked type. The thermoplastic elastomer of the noncrosslinked type ensures little product quality dispersion and lowered production cost because it is not subjected to any crosslinking reaction. However, in performance comparison, the olefinic thermoplastic elastomer of the crosslinked type is superior to the olefinic thermoplastic elastomer of the noncrosslinked type in respect of tensile strength and elongation at break or rubber properties (for example, elongation set and compression set) and heat resistance. This is widely known, which is described in detail in A. Y. Coran et al., Rubber Chemistry and Technology, vol. 53 (1980), p141.
The olefinic thermoplastic elastomer of the noncrosslinked or partially crosslinked type is described in a multiplicity of publications including Japanese Patent Publication Nos. 53(1978)-21021, 55(1980)-18448, 56(1981)-15741, 56(1981)-15742, 58(1983)-46138, 58(1983)-56575, 59(1984)-30376, 62(1987)-938 and 62(1987)-59139.
As mentioned above, the olefinic thermoplastic elastomer can be divided into the crosslinked type and the noncrosslinked type. With respect to the thermoplastic elastomer of the noncrosslinked type, it is desired to achieve the development of an olefinic thermoplastic elastomer composition which enables providing a molding being excellent in tensile strength, elongation at break, rubber properties (for example, elongation set and compression set), heat resistance and low temperature properties as compared with those of the conventional noncrosslinked thermoplastic elastomers. On the other hand, with respect to the thermoplastic elastomer of the crosslinked type, it is desired to achieve the development of an olefinic thermoplastic elastomer composition capable of providing a molding which is superior to the conventional vulcanized rubbers in low temperature properties, tensile strength, elongation at break and rubber properties.
Meanwhile, the molding of a polyolefin resin such as polyethylene or polypropylene has high rigidity and heat resistance, so that it finds a wide spectrum of uses.
Out of various polyolefin resins, however, polypropylene is crystalline and has high glass transition temperature, so that any polypropylene molding has poor impact resistance, especially, at low temperatures. Thus, the application field thereof has been limited.
For improving the impact resistance of the polypropylene molding, a method has been employed in which polypropylene is blended with polyethylene or a rubbery substance such as polyisobutylene, polybutadiene or an amorphous ethylene/propylene copolymer. Among the materials blended into polypropylene, frequent use is made of an amorphous or lowly crystalline ethylene/propylene random copolymer.
With respect to the composition composed of the above amorphous or lowly crystalline ethylene/propylene random copolymer and polypropylene, the impact resistance improving effect of the amorphous or lowly crystalline ethylene/propylene random copolymer is low, so that it must be blended into the polypropylene composition in a large amount for attaining a significant improvement of the impact resistance of the polypropylene molding.
Although the impact resistance of the molding from the polypropylene composition is significantly improved, the use of the ethylene/propylene random copolymer in a large amount causes another problem that the rigidity and heat resistance inherently possessed by polypropylene is gravely deteriorated. On the other hand, when it is intended to retain the rigidity and heat resistance inherently possessed by polypropylene by reducing the amount of ethylene/propylene random copolymer blended in the polypropylene composition, the problem that the improvement of the impact resistance at low temperatures is not satisfactory is raised.
Therefore, it is desired to achieve the development of a crystalline polyolefin resin composition which enables obtaining a polyolefin molding being excellent in not only rigidity and heat resistance but also impact resistance, especially, at low temperatures.
As apparent from the above, the olefinic thermoplastic elastomer and the crystalline polyolefin resin composition have the common problem of low temperature properties.